Magnetic filter device

ABSTRACT

In a magnetic filter apparatus in which permanent magnets are arranged to oppose each other with a container therebetween so as to generate a magnetic line of force in a direction substantially orthogonal to the moving direction of the fluid in the interior of the container, while regulating a filter passage time of the fluid in the range of 0.5 to 1.5 seconds, the permanent magnets are arranged so that the distance L (mm) between the permanent magnets in relation to the residual magnetic flux density B (T) of the permanent magnets satisfies the relationship: 
     
       
           B ×100≦ L≦B ×250  
       
     
     In this manner, the highest possible performance can be obtained from the filter using general-purpose permanent magnets such as ferrite or neodymium magnets, thereby achieving size reduction of the apparatus at low equipment cost.

TECHNICAL FIELD

The present invention relates to a magnetic filter apparatus forcontinuously separating magnetic particles contained in fluids, which isused in cleaning treatment of various types of fluid such as rolling oilfor cold-rolling steel sheets and washing liquids for removing therolling oil after the cold rolling.

BACKGROUND ART

In cleaning rolling oil for cold-rolling of steel sheets and washingliquids for removing the rolling oil remaining on the surface of thecold-rolled steel sheets, a magnetic filter apparatus is used to removemagnetic particles contained in the fluids.

A typical example of a conventional magnetic filter apparatus is nowexplained with reference to a cross-sectional view in FIG. 1(a) and aside view in FIG. 1(b). In the drawings, reference numeral 1 denotes acontainer, 2 denotes a permanent magnet, 3 denotes a filter element, 4denotes a back plate, 5 denotes a fluid inlet, and 6 denotes a fluidoutlet.

A ferromagnetic component comprising a metal grid composed of iron orferritic stainless steel such as SUS 430 is usually disposed as themagnetic filter element 3 in the interior of the container 1. At theexterior of the container 1, the permanent magnets 2 are arranged tooppose each other with the container 1 therebetween so as to generate amagnetic line of force in a direction substantially orthogonal to theflow direction of the fluid to be treated. The fluid to be treated isfed to the interior of the container 1 from the fluid inlet 5, passesthrough the magnetic filter element 3, and is discharged from the outlet6. Magnetic particles such as iron particles contained in the fluid tobe treated passing through the magnetic filter element 3 aremagnetically attracted to the magnetic filter element 3 magnetized bythe permanent magnets 2 and are separated from the fluid to be treated.

In the above-described capturing of the magnetic particles using themagnetic filter apparatus, the attractive force Fm of the filaments ormetal grid constituting the filter element is expressed by the formula:

Fm=χ·V·H·(dH/dx),

wherein

χ: magnetic susceptibility of the particles,

V: volume of the particles,

H: intensity of the magnetic field, and

dH/dx: magnetic gradient (spatial variation in the magnetic field.

In the above formula, χ and V are inherent properties of the magneticparticles. Thus, in order to increase the attractive force Fm andimprove the performance of the filter, either the magnetic field H orthe magnetic gradient dH/dx must be increased. However, the magneticgradient dH/dx is a coefficient dependent on the material and the shapeof the ferromagnetic component which constitutes the filter element;accordingly, after the material and the shape of the ferromagneticcomponent are determined, the magnetic gradient dH/dx is regulated bythe intensity of the magnetic field. Thus, the foremost requirement forimproving the performance of the filter, i.e., the attractive power, isto sustain a strong magnetic field in the interior of the filter.

Hitherto, the relationship between the performance of the filter and themagnetic field has not been fully examined. Accordingly, failures suchas degradation of the performance of the filter due to a diminishedmagnetic field in the filter have occurred frequently. As for theselection of the magnets, it is not clear what degree of strength isrequired from a magnet in order to achieve the desired filterperformance. Moreover, because the relationship between the shape of thefilter, the flow speed of the fluid to be treated, and the strength ofthe magnet is not clear, the filter cannot achieve the desiredperformance.

In other words, strong magnets do not always yield satisfactory resultsbecause of their design and specifications.

Moreover, the use of strong magnets increases the equipment cost,although some improvement can be expected.

DISCLOSURE OF INVENTION

The present invention favorably solves the above-described problems. Anobject of the present invention is to provide a magnetic filterapparatus of reduced size at low cost by yielding the highest possibleperformance from the filter in which general-purpose permanent magnetssuch as ferrite or neodymium magnets are used.

In order to clarify the relationship between the intensity of themagnetic field of the magnetic filter apparatus and the performance ofthe filter, the present inventors have conducted research on theinfluence of the various factors on the performance of the filter.During the course, the present inventors have succeeded in clarifyingthe effect of the various factors on the performance of the filter anddeveloped a low-cost high-efficiency magnetic filter apparatus based onthis finding.

That is, the present invention is a magnetic filter apparatuscomprising: a container having an inlet and an outlet for fluid; afilter element comprising a ferromagnetic material disposed in thecontainer; and permanent magnets for magnetizing the filter element, thepermanent magnets being arranged to oppose each other with the containertherebetween so as to generate a magnetic line of force in a directionsubstantially orthogonal to the moving direction of the fluid inside thecontainer,

wherein, while regulating a filter passage time of the fluid in therange of 0.5 to 1.5 seconds, the permanent magnets are arranged so thatthe distance L (mm) therebetween in relation to the residual magneticflux density B (T) of the permanent magnets satisfies the relationship:

B×100≦L≦B×250

In the present invention, the permanent magnets for magnetizing thefilter element preferably have a residual magnetic flux density of 0.4 Tor more.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating a typical example of a known magneticfilter apparatus in cross-section in (a) and by side view in (b).

FIG. 2 is a graph showing the effects of the residual magnetic fluxdensity B (T) of the permanent magnets and the distance L (mm) betweenthe permanent magnets on the iron particle separation rate η.

FIG. 3 is a graph showing the relationship between the distance betweenthe magnets, the ratio of the residual magnetic flux densities (L/B),and the equipment cost of the filter.

FIG. 4 is a graph showing the relationship between the distance Lbetween the magnets and the residual magnetic flux density B of thepermanent magnets capable of yielding a satisfactory iron particleseparation rate.

FIG. 5 is a graph showing the relationship between the performance ofthe filter (the iron particle separation rate η) per unit and theequipment cost of the filter.

FIG. 6 is a diagram describing a filter length A and a flow speed v inthe filter.

FIG. 7 is a graph showing the relationship between a filter passage timet and the iron particle separation rate η.

FIG. 8 is a graph showing the relationship between the filter passagetime t and the equipment cost of the filter.

FIG. 9 is a diagram illustrating a cleaning system incorporating amagnetic filter apparatus of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention is described below by way of an embodiment.

First, the course of arriving at the present invention is explained.

The following factors have been considered to affect the performance ofa filter:

the strength of magnets;

the distance between the magnets;

the material and the shape of a filter element;

the flow speed;

the length of the filter element; and

the characteristics of the fluid.

In examining these factors related to the performance of the filter, ametal grid of a commonly used ferritic stainless steel SUS 430 (mesh 10,wire: 1.0 mm dia.) was placed in the container as the filter element. Analkaline washing liquid commonly employed for cleaning cold-rolled steelsheets was used as the fluid. The alkaline washing liquid, usuallyrecyclable, had an inlet iron particle concentration of approximately 60mass ppm to approximately 100 mass ppm before being treated by thefilter.

The performance of the filter was evaluated according to the formula:

iron particle separation rate η=(F−E)/F×100 (%)

wherein F represents the inlet iron particle concentration and Erepresents the outlet iron particle concentration.

The performance of the filter is assumed to be satisfactory if the ironparticle separation rate η is 60% or more. On the other hand, an ironparticle separation rate η of less than 60% is not consideredsatisfactory since, as described below, the volume of the circulatingflow must be increased in order to secure cleanliness of the fluid,thereby requiring large-scale filter equipment.

In the examination of the performance of the filter, the iron particleseparation rate η was examined for specimens sampled 10 to 20 minutesafter backwashing of the filter when filtering was stably performed.

Commonly-employed ferrite or neodymium magnets having a residualmagnetic flux density B of approximately 0.2 T to approximately 0.6 Twere used as the permanent magnets.

The distance L between the permanent magnets shown in FIG. 1(a) iscrucial for obtaining the desired performance from the magnetic filterapparatus. In this respect, the iron particle separation rate η wasmeasured while varying the distance L between the magnets from 35 mm to200 mm.

FIG. 2 shows the experimental results of the effect of the residualmagnetic flux density B (T) of the employed permanent magnets and thedistance L (mm) between the magnets on the iron particle separation rateη. Note that the time taken for the fluid to pass through the filter wasset at 1.0 second.

As is apparent from the graph, the filter stably exhibits excellentperformance when the residual magnetic flux density B (T) and thedistance L (mm) between the magnets satisfy the formula:

L≦250×B

Next, the experiment was conducted by reducing the distance L betweenthe magnets. At a distance L of less than B×100, although the ironparticle separation rate η is maintained at a high level, thecross-sectional area of the filter reduced remarkably. Accordingly, alarge number of filter units are necessary to secure the volume of thecirculating flow, which would result in a complicated system, cumbersomemaintenance, and significantly high equipment cost.

The equipment cost for the filter was examined by varying L/B usingactual equipment for alkali-washing rolled steel sheets. The volume ofthe washing liquid for the steel sheets was approximately 20 m³ and thecirculating flow was 0.2 m³/min. The results are shown in FIG. 3. In thegraph, the equipment costs are compared relative to the equipment costat L/B=150, which is defined as 1.0.

As is apparent from the graph, a decrease in L/B causes an increase inthe equipment cost because the number of filters required for securingthe volume of the circulating flow must be increased, although the ironparticle separation performance of the filter is improved. Especiallywhen L/B is less than 100, the equipment cost drastically increases.

Accordingly, in the present invention, as shown in FIG. 4, the residualmagnetic flux density B of the permanent magnets and the distance Lbetween the magnets are set to satisfy the relationship:

100×B≦L≦250×B

Note that in the above-described experiment, the iron particleconcentration of the fluid at the inlet of the filter was approximately60 mass ppm to 100 mass ppm. However, since the filter is constantlyrecycled, the target cleanliness of the circulating fluid is usually 30mass ppm or less.

The relationship between the performance (iron particle separation rateη) of the filter per unit and the equipment cost for the filter wasexamined using actual alkali-washing equipment for rolled steel sheets.In the experiment, a filter having a circulating flow volume of 0.2m³/min was installed onto the path of the alkaline washing liquid tomaintain the iron particle concentration in the alkaline washing liquidat approximately 20 ppm. The volume of washing liquid for the steelsheets was approximately 20 m³, and the average iron particleconcentration at the inlet of the filter was approximately 150 mass ppm.The results are shown in FIG. 5.

In the graph, the equipment cost is compared relative to the equipmentcost required at an iron particle separation rate η of 70%, which isdefined as 1.0.

As shown in the graph, at an iron particle separation rate η per unit ofless than 60%, a large-scale filter is required to maintain the desiredcleanliness of the washing liquid, resulting in high equipment cost.Thus, the iron particle separation rate η of the filter should be 60% ormore also from the point of view of equipment cost efficiency.

Next, the flow volume, the flow speed, and the passage time taken forthe fluid to be treated to pass through the filter were examined. Theflow speed of the fluid to be treated was varied from 100 mm/sec to 300mm/sec. The iron particle separation rate η was measured at a filterpassage length of 50 mm, 100 mm, 150 mm, and 200 mm. FIG. 6 shows thefilter length A and the flow speed v of the fluid in the filter. Hereinthe filter passage time t is:

t=A/v

wherein

t: the time taken for the fluid to pass through the filter (sec),

A: length of the filter (mm), and

v: flow speed of the fluid in the filter (mm/sec).

The above-described experiment demonstrates that the performance of thefilter, i.e., the iron particle separation rate η, can be organized interms of the filter passage time.

In FIG. 7, the results of the examination on the relationship betweenthe filter passage time t and the iron particle separation rate η areorganized.

As shown in the graph, in all the samples, the iron particle separationrate η drastically decreased and the performance of the filter wassignificantly degraded at a filter passage time t of less than 0.5seconds. Moreover, no significant improvements were observed at a filterpassage time t exceeding 1.5 seconds.

Next, the relationship between the filter passage time t and theequipment cost for the filter was examined in actual alkali-washingequipment for rolled steel sheets. In the experiment, the volume of thewashing liquid for steel sheets was approximately 20 m³ and the averageiron particle concentration at the inlet of the filter was approximately150 mass ppm in the path for the alkaline washing liquid. The filter wasinstalled onto the path in such a manner that the iron particleseparation rate η was 70% at a circulating flow volume of 0.2 m³/min anda passage time of 1.0 second so as to maintain the iron particleconcentration in the alkaline washing liquid at approximately 20 massppm. The results are shown in FIG. 8. In the graph, the equipment costis compared relative to the equipment cost at the filter passage timet=1.0 second, which is defined as 1.0.

As shown in the graph, at a filter passage time t exceeding 1.5 seconds,although the necessary iron particle separation rate can be obtained ata small residual magnetic flux density of the permanent magnets and alarge distance between the magnets, a large-scale filter is required tomaintain the cleanliness of the washing liquid, resulting in increasedequipment cost. Thus, the filter passage time t should be 1.5 seconds orless from the point of view of equipment efficiency.

The results shown in FIGS. 7 and 8 demonstrate that the effective filterpassage time t is in the range of 0.5 to 1.5 seconds considering theperformance of the filter and the equipment cost.

Accordingly, in the present invention, the filter passage time of thefluid is limited to the range of 0.5 to 1.5 seconds.

EXAMPLES

Cleaning treatment of the washing liquid was performed using magneticfilter apparatuses of the present invention in actual cleaning equipmentshown in FIG. 9.

As shown in the drawing, a steel sheet 7 after rolling was passedthrough a rough washing tank 8, usually called a dunk-tank, brushed by afirst brush scrubber 9, and subjected to main washing in a cleaning tank10.

The dunk tank 8 and the cleaning tank 10 were provided with circulatingtanks 11 and 12, respectively, and a washing liquid mainly constitutingan alkaline washing liquid was circulated using pumps 13 and 14.

The washing liquid in the circulating tanks 11 and 12 was fed tomagnetic filter apparatuses 15 and 16 using pumps 17 and 18,respectively, to attract and separate the iron particles removed fromthe steel sheets during cleaning.

The specifications of the magnetic filter apparatus 16 for thecirculating tank of the cleaning tank, the filter passage time of thewashing liquid, and the iron particle concentration at the inlet areshown in Table 1.

Under the above-described conditions, the iron particle concentration ofthe washing liquid at the outlet after the cleaning treatment of thewashing liquid and the iron particle separation rate η were examined.The results are also shown in Table 1.

As shown in the table, the iron particle separation rate η was 60% ormore when the magnetic filter apparatus of the present invention is usedin the treatment, achieving satisfactory results.

The examination was also conducted for the cleaning treatment using themagnetic filter apparatus of the present invention as the magneticfilter apparatus 15 for the circulating tank of the dunk tank. Theobtained results were satisfactory.

EFFECT OF THE INVENTION

In the cleaning treatment of the fluid using general-purpose permanentmagnets, the present invention yields the highest possible performancefrom the filter, thereby achieving size reduction with low equipmentcost.

Conventionally, during continuous annealing after washing, residual ironparticles from the surface of steel sheets adhere onto the surface ofthe rollers in the furnace, thereby frequently generating irregularitydefects known as roll marks. This results in degradation in theproduction yield of approximately 0.2 to 0.5%. However, by using themagnetic filter apparatus of the present invention in the cleaningtreatment, the iron particles can be powerfully and stably removed, andsuch defects can be eliminated thereby.

TABLE 1 Residual Iron Particle Iron Particle Iron Magnetic Flux DistanceFilter Concentration Concentration Particle Density of between FilterPassage at Fluid at Fluid Separation Permanent Magnets Length Time InletOutlet Rate No. Magnets (T) L (mm) A (mm) t (sec) (mass ppm) (mass ppm)η (%) 1 0.6 150  200 1.5 80 20 75 2 0.6 150  100 1.0 70 22 69 3 0.6 150  50 0.5 76 30 61 4 0.6 90 200 1.5 74 11 85 5 0.6 90 100 1.0 68 15 78 60.6 90  50 0.5 91 27 70 7 0.4 90 150 1.5 95 23 76 8 0.4 90 150 1.0 66 2070 9 0.4 90 150 0.5 73 27 63 10 0.4 50 150 1.5 87 12 86 11 0.4 50 1501.0 88 16 82 12 0.4 50 150 0.5 76 19 75

What is claimed is:
 1. A magnetic filter apparatus comprising: acontainer having inlet and outlet for fluid; filter element comprisingferromagnetic material disposed in the container; and permanent magnetsfor magnetizing the filter element, the permanent magnets being arrangedto oppose each other with the container therebetween so as to generate amagnetic line of force in a direction substantially orthogonal to themoving direction of the fluid inside the container, wherein, whileregulating filter passage time of the fluid in the range of 0.5 to 1.5seconds, the permanent magnets are arranged so that the distance L (mm)therebetween in relation to the residual magnetic flux density B (T) ofthe permanent magnets satisfies the relationship: B×100≦L≦B×250.